This invention relates to apparatus producing a machine-direction heat seal for sealing a hem in a moving web of film and more particularly to the manufacture of draw tape plastic bags.
Bags made of thin polyethylene materials have been used in various sizes. Small bags are used in the packaging of samples and the like. Larger bags are used as shopping bags; and even larger bags are used for containing trash.
A particularly advantageous closure for such a bag includes a draw tape constructed from the same polyethylene material. U.S. Pat. No. 3,029,853--Piazzi and British patent No. 1,125,363--Jortikka are examples of draw tape bags. Such closures have been successfully employed on these bags.
Draw tape closures for large trash bags, and the manufacture of these draw tape trash bags, are described in the related applications identified below.
Forming the hem, into which the tape is inserted, is shown for example, in U.S. Pat. Nos. 2,897,729--Ashton et al 3,058,402--Kugler, 3,058,403--Kugler, and in copending patent applications of applicant's assignee including "METHOD AND APPARATUS FOR FORMING HEMS IN SUPERPOSED PLIABLE PANELS," Boyd et al, Ser. No. 652,255, filed Sept. 20, 1984, now U.S. Pat. No. 4,617,008; and in "HEM FOLDER WITH INTEGRAL TAPE INSERTER FOR MAKING DRAW TAPE BAGS," Herrington, Ser. No. 871,238 filed June 6, 1986.
Draw tape hems have been previously made by various methods. One method is to use a Teflon-coated hot bar which opens and closes against the film in the area of the bag machine where the film starts and stops. The bar closes once during each cycle while the film is stopped. A second method is to blow a series of hot air jets against the film as it travels by. Bag closures for use in the field use a pair of heated belts to produce a machine-direction seal. In the hot bar method, the hot bar is located far downstream from where the tape is inserted into the hem and also where the hem is folded, so there has been an opportunity for the tape and the hem to wander by the time it reaches the sealer. This requires making the hem wider to allow for this wandering movement. This is costly in wasted material, produces a weak tape seal at the side edges of the bag, and also makes a sloppy bag. The hot air sealer can be used on a continuously moving web, but it is very sensitive to wrinkles. The heating of the film by the air jets also causes some wrinkling which cannot easily be prevented since the film is not mechanically constrained. As a result the hem seals made by these techniques are not reliable, particularly where thin films, in the order of one mil, are used.
It is an object of the present invention to seal a hem in a web of film moving at a relatively high speed, such as 250-300 ft./min, where the seal in the hem is produced as close as possible to where the hem is folded and the tape inserted, and while the film is still held flat on the folding surface so there are no wrinkles in the hem.
It is a further object of the present invention to seal hems in the opposing panels of a moving web of folded thermoplastic film in the manufacture of draw tape bags.